Why is a sitting blood pressure reading higher than a standing blood pressure reading?

Gravity: The short answer is the effect of gravity. When you stand up, your blood will "pool" in your lower extremities. The body has to increase the heart rate and vascular tone in order to keep blood pumping upwards towards your arms, neck and head. It is typical for the blood pressure to fall a little bit when you stand up; if it falls too much, this can be abnormal though, and should be evaluated.